1. Field of the Invention
This invention relates to a knock detection apparatus, and more particularly to a knock detection apparatus for calculating a threshold value for knock determination based on a detected knock signal from an internal combustion engine.
2. Description of the Related Art
There has been proposed a knock control method in which knock having occurred in an engine is detected, and ignition timing is controlled based on a detected knock condition (see e.g. Japanese Unexamined Patent Publication No. 08-218996). In this knock control method, only a frequency component which can contain vibration caused by engine knock is obtained by filtering an output from a knock sensor using a band pass filter (BPF), and a peak value of the frequency component is sampled whenever the TDC (top dead center) position of each cylinder is reached. Then, the sampled peak value is compared with an averaged value (hereinafter referred to as “background”) of peak values sampled so far, and whether or not knock has occurred is determined based on whether the currently sampled peak value is higher than a predetermined level above the background, whereby the amount of ignition advance is feedback-controlled. The feedback control is performed such that if it is determined that knock has not occurred, ignition timing is progressively advanced, whereas if it is determined that knock has occurred, ignition timing is retarded.
By the way, the background is obtained by averaging sampled peak values as mentioned above. More specifically, a value obtained by subtracting the immediately preceding background from the current peak value is multiplied by a predetermined coefficient, and a value obtained by the multiplication is added to the immediately preceding background to thereby determine the current background, this value (updated value) being provided with an update guard value for preventing a sudden change of the background due to noise. Therefore, it takes time for the background to converge to a constant value at which the engine is started.
FIG. 34 is a diagram useful in explaining the convergence of background during the start of the engine. As shown in FIG. 34, sampling of peak values is started upon the start of the engine. However, since the background is obtained by averaging sampled peak values, it is updated whenever the TDC position of each cylinder is reached, whereby the background is caused to progressively converge to a peak value. For this reason, if knock determination is performed before the background has converged to the peak value (i.e. during a period F in FIG. 34), it can be determined that the peak value is higher than the predetermined level above the background without occurrence of knock, which causes erroneous determination that knock has occurred. To prevent this erroneous determination, it is a conventional practice to start the knock feedback control after the lapse of a predetermined time period, by taking a time period required for convergence of the background into consideration.
Further, although in the calculation of the background, the update guard value for preventing a sudden change of the background due to noise is provided as mentioned hereinabove, if a sudden change of the peak value is caused not by noise, but by a correct cause, the update guard value retards convergence of the background. To eliminate this inconvenience, it is conventional to change the update guard value according to engine speed and intake pipe pressure so as to prevent retardation of convergence of the background. For example, when engine speed or intake pipe pressure rises suddenly, the absolute value of the peak value also increases suddenly. In this case, since the change in the peak value is not due to noise, the update guard value is increased so as to enable the background to quickly converge to the peak value. This causes the background to converge to a predetermined value.
As described above, in the prior art, even when the background has actually converged to a peak value, knock feedback control is started after the lapse of a predetermined time period, resulting in a narrowed knock control range.
Further, at the start of the engine, it takes time for the background (threshold value) to converge, inhibiting knock determination from being performed before the background has converged.
Furthermore, in the prior art, the update guard value (upper limit value) is changed only based on engine speed and intake pipe pressure, but not changed even when the peak value has changed due to other factors except noise. Therefore, in the latter case, the convergence of the threshold value is retarded.